1. Technical Field
The present invention relates to the field of radomes, and more particularly to efficient radomes of variable geometry.
2. Description of the Related Art
Conventional radomes are typically dome-like shells that can be used to protect enclosed electromagnetic devices, such as antennas, from environmental conditions, such as wind, solar loading, ice, and snow. Radomes, such as a solid laminate and sandwich radomes, can be rigid self-supporting structures. Mismatches between the impedance of free space and the radome can result in energy dissipation at the point of incidence. The energy dissipation can be the result of a reflective wave being generated at a medium boundary, such as the radome/free-space boundary.
If an electromagnetic wave strikes a medium boundary at a point which is multiple of a half wavelength, energy dissipation at the boundary can be minimized. A material which minimizes reflections across medium boundaries by ensuring electromagnetic incidence occurs at half-wavelength multiples for a selected frequency utilizes an impedance transform. Advantageous transfer characteristics for conventional radomes are generally achieved through such a wavelength dependant impedance transform. More particularly, half-wavelength transforms can be advantageously used to achieve beneficial transfer characteristics.
Relying upon such an impedance transform, however, results in radomes optimized for specific frequencies and places a limitation upon radome thickness. The further the deviation from the optimized frequency, the greater the perturbations caused by the exemplary conventional radome; since the half-wavelength transform cannot properly function for differing wavelengths. Consequently, conventional radomes are frequency dependant.
Differing angles of incidence also substantially affect the transfer characteristics of conventional radomes. Different angles of incidence cause waves to travel different distances through a uniformly thick medium. For example, a wave at normal incidence passing through a 1.5 cm thick medium travels 1.5 cm.                distance=thickness/sin(incident angle), so that        distance=1.5 cm/sin 90=1.5 cm/1=1.5 cm        
Alternately, a wave at a 30 degree incident angle passing through the same medium (ignoring refraction) travels a distance of 3.0 cm.                distance=thickness/sin(incident angle), so that        distance=1.5 cm/sin 30=1.5 cm/0.5=3.0 cm        
Consequently, performance of conventional radomes is significantly affected by various incident angles.
To minimize differences in incident angles, conventional radomes are often hemispherically shaped. Accordingly, if a radio frequency source is centrally placed within a hemispherical radome, waves generated by the source will strike the radome boundary at a substantially normal angle of incidence. Other shapes would result in differing angles of incidence, thereby degrading radome performance characteristics.
A number of difficulties result from the necessity that conventional radomes be hemispherically shaped. For example, manufacturing and transportation considerations cause most large conventional radomes to be formed from multiple-curved panels that can be joined on-site to form the radome structure. The coupling planes at which adjacent panels are joined, however, can cause thickness variations. The thickness variations can result in decreased radome performance at the coupling planes—the coupling planes being the seams in a radome wall existing between joined radome panels. To minimize loss at panel boundaries, panels are made as large as practicable for a given situation. It can be very difficult to transport, install, and manufacture the large, rigid, and curved radome panels.
Another negative aspect of conventional radomes relates to radome frames. A radome frame is a supporting framework that provides mechanical support to a radome. Such additional support can be necessary since radome walls, which utilize wavelength dependant impedance transforms, are thickness restricted, generally to multiples of half a wavelength of an optimized frequency. Conventional radomes can require support greater than that provided by material which is half a wavelength thick.
For example, a large radome, such as the 140-foot diameter radome at Mt. Hebo, may need to be constructed of a dielectric material thicker than the lowest half wavelength, which would be 1.5 cm for a 10 GHz frequency. Increasing thickness of a radome wall to the next higher half wavelength multiple can significantly increase the cost to manufacture the radome wall. Additionally, increased losses due to the magnetic and electric loss tangents occur as the thickness of a radome increases. Accordingly, load bearing radome frames are often used in conjunction with radome walls.
Losses attributable to radio frequency waves striking radome frames can be called scatter loss. Scatter loss of conventional radomes with radome frames can be as great as 10 times the wall pass loss. While many different approaches have been taken to minimize scatter loss, scatter loss remains a significant problem for conventional radomes with radome frames.